Zeolites, inorganic films

This method is particularly important, and is used widely in the preparation of zeolites, thin films and abrasive coatings. The process involves preparation of a concentrated solution (sol) of the reaction components. This mixture is then converted to an almost rigid gel by removing the solvent or by adding a component which causes the gel to solidify. The gel can then be treated in some way to produce the desired material. This methodology can also be used to produce composite materials, where organic and inorganic species can be trapped in an inert matrix. The properties of both the inert matrix and the trapped species are combined in the product. 

The separation factors are relatively low and consequently the MR is not able to approach full conversion. With a molecular sieve silica (MSS) or a supported palladium film membrane, an (almost) absolute separation can be obtained (Table 10.1). The MSS membranes however, suffer from a flux/selectivity trade-off meaning that a high separation factor is combined with a relative low flux. Pd membranes do not suffer from this trade-off and can combine an absolute separation factor with very high fluxes. A favorable aspect for zeoHte membranes is their thermal and chemical stability. Pd membranes can become unstable due to impurities like CO, H2S, and carbonaceous deposits, and for the MSS membrane, hydrothermal stability is a major concern [62]. But the performance of the currently used zeolite membranes is insufficient to compete with other inorganic membranes, as was also concluded by Caro et al. [63] for the use of zeolite membranes for hydrogen purification. 

Zeolite/polymer mixed-matrix membranes can be fabricated into dense film, asymmetric flat sheet, or asymmetric hollow fiber. Similar to commercial polymer membranes, mixed-matrix membranes need to have an asymmetric membrane geometry with a thin selective skin layer on a porous support layer to be commercially viable. The skin layer should be made from a zeohte/polymer mixed-matrix material to provide the membrane high selectivity, but the non-selective porous support layer can be made from the zeohte/polymer mixed-matrix material, a pure polymer membrane material, or an inorganic membrane material. 

Different methods have been used to deposit microporous thin films, including solgel, pyrolysis, and deposition techniques [20], Porous inorganic membranes are made of alumina, silica, carbon, zeolites, and other materials [8], They are generally prepared by the slip coating method, the ceramic technique, or the solgel method (Section 3.7). In addition, dense membranes are prepared with metals, oxides, and other materials (Chapter 2). 

ZSM-5 was grown on the outer surface of a porous alumina tube by Matsuda et al. [45]. The tube was first sealed with an inorganic adhesive to avoid crystallization on the inner walls and subsequently vertically positioned in a ZSM-5 synthesis solution. Under stirring conditions at 200°C after 48 hours, a film was formed of ZSM-5 on the outer surface. The synthesis was repeated several times to obtain a 100 pm thick and dense layer. Finally, the zeolite composite phase was calcined. 

There are two important functions of the pore system, beside the provision of an acceptable texture. One is the heat transfer during baking, which takes place by evaporation and condensation over the pore network. The pore curvature will directly influence this process. The second function is the accumulation of flavour compounds formed by Maillard (browning) reactions during the final period in the oven. Non-polar lipids form a surface film in the pore system, where these flavour compounds are absorbed - just as porous inorganic materials such as zeolites adsorb incoming species. (The relation between absorption and curvature was discussed in Chapter 2.)... 

Nano-technology will play a prominent role in the future synthesis of molecular thin films and devices. Nano-technology is defined as the study and manufacture of structures and devices with dimensions about the size of a molecule. Nano-scale physics and chemistry might lead directly to the smallest and fastest transistors and the strongest and lightest materials ever made [2], Likewise, bio-catalysts such as proteins will be increasingly used to facilitate relevant chemical reactions at ambient conditions. Natural macromolecules will be explored to provide selectivity similar to inorganic chemicals such as zeolites. 

Defect-free zeolite films are now available for separations (also see Chap. 6). They are usually made on a porous inorganic support.184 These have been used to separate benzene over p-xylene by an a-valve of more than 160, n-butane over isobutane by one of 88,185 water over propanol one of 71,186 hydrogen over isobutane with one of 151, hydrogen over methane with one of 500,187 and hydrogen... 

Different types of inorganic materials, such as metal oxides, clays, and zeolites, can also be deposited on electrode surfaces. Such films are of interest, because they frequently show well-defined structures (e.g., they have unique pore or interlayer sizes), they are thermally and chemically very stable, and are usually inexpensive and readily available. A few examples will be described. 

Other inorganic antiblocking fillers such as calcium carbonate, alumina-silicate ceramic spheres, zeolite, kaolin day, feldspar, and mica have also been used as antiblocks. In PE film, caldum carbonate can provide the low blocking force of talc and DE, but only when loaded at 2-3 times their concentration (or higher), reducing darity of the film, and increasing its density [12-1, 12-22, 12-30]. 

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